Hollow fiber membranes are used in a wide variety of applications. Ultrafiltration hollow fiber membranes are used to separate proteins and other macromolecules from aqueous solutions. Ultrafiltration membranes are usually rated in terms of the size of the solute they will retain. Typically, ultrafiltration membranes can be produced to retain dissolved or dispersed solutes of from about 1000 Daltons to about 1,000,000 Daltons. They can be rated by Molecular Weight Cutoff, which is the molecular weight expressed in Daltons, a unit of molecular mass, at which a stated percent of the feed concentration of the solute being processed is retained or rejected by the membrane. Manufacturers usually set the stated percent at 90% to 95%. Ultrafiltration membranes can also be designated by their average or nominal pore size. The nominal or average pore size of typical ultrafiltration membranes is in the range of about 2 nanometers to about 50 nanometers.
Hollow fiber membranes are also used as membrane contactors, typically for degassing or gas absorption applications. Contactors bring together two phases, i.e., two liquid phases, or a liquid and a gas phase for the purpose of transferring a component from one phase to the other. A common process is gas-liquid mass transfer, such as gas absorption, in which a gas or a component of a gas stream is absorbed in a liquid. Liquid degassing is another example, in which a liquid containing dissolved gas is contacted with an atmosphere, a vacuum or a separate phase to remove the dissolved gas. In an example of conventional gas absorption, gas bubbles are dispersed in an absorbing liquid to increase the gas/liquid surface area and increase the rate of transfer of the species to be absorbed from the gas phase. Conversely, droplets of liquid can be sprayed or the liquid can be transported as a thin film in counter-current operation of spray towers, packed towers, etc. Similarly, droplets of an immiscible liquid can be dispersed in a second liquid to enhance transfer. Packed columns and tray columns have a deficiency as the individual rates of the two phases cannot be independently varied over wide ranges without causing flooding, entrainment, etc. If however, the phases are separated by a membrane, the flow rates of each phase can be varied independently. Furthermore, all the area is available, even at relatively low flow rates. Due to these advantages, hollow fiber membranes are increasingly being used in contactor applications.
Hydrophobic microporous membranes are commonly used for contactor applications with an aqueous solution that does not wet the membrane. The solution flows on one side of the membrane and a gas mixture at a lower pressure than the solution flows on the other. Pressures on each side of the membrane are maintained so that the liquid pressure does not overcome the critical pressure of the membrane, and so that the gas does not bubble into the liquid. Critical pressure, the pressure at which the solution will intrude into the pores, depends directly on the material used to make the membrane, inversely on the pore size of the membrane, and directly on the surface tension of the liquid in contact with the gas phase. Hollow fiber membranes are primarily used because of the ability to obtain a very high packing density with such devices. Packing density relates to the amount of useful filtering surface per volume of the device. Also, they may be operated with the feed contacting the inside or the outside surface, depending on which is more advantageous in the particular application. Typical applications for contacting membrane systems are to remove dissolved gases from liquids, “degassing”; or to add a gaseous substance to a liquid. For example, ozone is added to very pure water to wash semiconductor wafers.
Porous contactor membranes are preferred for many applications because they will have higher mass transfer than nonporous membranes. For applications with liquids having low surface tensions, smaller pore sizes will be able to operate at higher pressures due to their resistance to intrusion. Non-porous contactor membranes are preferred in cases where the liquid vapor pressure is high, or where high temperature operation increases the vapor pressure. In these cases, evaporation through a porous membrane may result in substantial liquid loss. Non-porous membranes may also be preferred in high pressure applications, where intrusion of a porous membrane would be a problem. Furthermore, in applications where a liquid phase has a surface tension of less than about 20 mN/m, (milliNewtons per meter), a nonporous membrane would be advantageous as porous membranes would be intruded by such low surface tension liquids.
A hollow fiber porous membrane is a tubular filament comprising a outer diameter, an inner diameter, with a porous wall thickness between them. The inner diameter defines the hollow portion of the fiber and is used to carry fluid, either the feed stream to be filtered through the porous wall, or the permeate if the filtering is done from the outer surface. The inner hollow portion is sometimes called the lumen.
The outer or inner surface of a hollow fiber membrane can be skinned or unskinned. A skin is a thin dense surface layer integral with the substructure of the membrane. In skinned membranes, the major portion of resistance to flow through the membrane resides in the thin skin. The surface skin may contains pores leading to the continuous porous structure of the substructure, or may be a non-porous integral film. Asymmetric refers to the uniformity of the pore size across the thickness of the membrane; for hollow fibers, this is the porous wall of the fiber. Asymmetric membranes have a structure in which the pore size is a function of location through the cross-section. Another manner of defining asymmetry is the ratio of pore sizes on one surface to those on the opposite surface.
Manufacturers produce membranes from a variety of materials, the most general class being synthetic polymers. An important class of synthetic polymers are thermoplastic polymers, which can be flowed and molded when heated and recover their original solid properties when cooled. As the conditions of the application to which the membrane is being used become more severe, the materials that can be used becomes limited. For example, the organic solvent-based solutions used for wafer coating in the microelectronics industry will dissolve or swell and weaken most common polymeric membranes. The high temperature stripping baths in the same industry consist of highly acidic and oxidative compounds, which will destroy membranes made of common polymers. Perfluorinated thermoplastic polymers such as poly(tetrafluoroethylene-co-perfluoro(alkylvinylether)) (Poly(PTFE-CO-PFVAE)) or poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) are not adversely affected by severe conditions of use, so that membranes made from these polymers would have a decided advantage over ultrafiltration membranes made from less chemically and thermally stable polymers. These thermoplastic polymers have advantage over poly(tetrafluoroethlene) (PTFE), which is not a thermoplastic, that they can be molded or shaped in standard type processes, such as extrusion. Using the process of the present invention, hollow fiber membranes can be produced at smaller diameters than possible with PTFE. Smaller diameters are useful in compact equipment, such as in aerospace applications.
Being chemically inert, the poly (PTFE-CO-PFVAE) and FEP polymers are difficult to form into membranes using typical solution casting methods. They can be made into membranes using the Thermally Induced Phase Separation (TIPS) process. In one example of the TIPS process a polymer and organic liquid are mixed and heated in an extruder to a temperature at which the polymer dissolves. A membrane is shaped by extrusion through an extrusion die, and the extruded membrane is cooled to form a gel. During cooling the polymer solution temperature is reduced to below the upper critical solution temperature. This is the temperature at or below which two phases form from the homogeneous heated solution, one phase primarily polymer, the other primarily solvent. If done properly, the solvent rich phase forms a continuous interconnecting porosity. The solvent rich phase is then extracted and the membrane dried.
An advantage for contacting applications is that the very low surface tension of these perfluorinated polymers allows use with low surface tension liquids. For example, highly corrosive developers used in the semiconductor manufacturing industry may contain surface tension reducing additives, such as surfactants. These developers could not be degassed with typical microporous membranes because the liquid would intrude the pores at the pressures used and permeate, causing solution loss and excess evaporation. In addition, liquid filling the pores would greatly add to the mass transfer resistance of gas transport. U.S. Pat. No. 5,749,941 describes how conventional hollow fiber membranes of polypropylene or polyethylene cannot be used in carbon dioxide or hydrogen sulfide absorption into aqueous solutions containing an organic solvent without the use of an solution additive to prevent leakage. While PTFE membranes would work in these applications, presumably because of their lower surface tension, they are difficult to process into hollow fibers. The membranes of the present invention are made from polymers having similar surface tension properties to PTFE and are more readily manufactured into small diameter hollow fiber membranes.
Ultrafiltration membranes are primarily manufactured as skinned asymmetric membranes because this structure gives the advantage of high permeation rates for the small pores needed for efficient retention of solutes. In skinned asymmetric membranes, the pores required for solute retention are produced only in the surface skin. This is done to offset the high resistance to flow inherent to pores of the diameter common to ultrafiltration membranes by reducing the length of the pores, i.e., the thickness of the skin. This is a significant difference from microporous membranes, which are usually not skinned, and usually have a symmetric pore structure through the membrane cross-section. The larger pore size of microporous membranes is sufficient to have economically feasible permeation rates even for membranes with a uniform pore size through the total membrane thickness. Membranes having the pore size of ultrafiltration membranes and having a symmetric pore size through the membrane thickness would have very low permeation rates. Similarly, contactor membranes with pore sizes of the same size as ultrafiltration membranes would have increased mass transfer resistance unless they were made as asymmetric skinned membranes, where the small pores were only in the skin.
Asymmetric skinned hollow fibers are primarily used with the skin on the inner lumen. The present invention describes such membranes and a process to produce them. The process described has also been adapted to produce an asymmetric hollow fiber membrane with an outside skin. The inventors of this method found that using very short air gaps, the distance from the exit of the die tip to the cooling bath surface, they were able to control the thickness of the skin that formed when solvent evaporated from the outer surface. Previous microporous poly(PTFE-CO-PFVAE) and FEP membranes made from the TIPS method required extrusion through an air gap. poly(PTFE-CO-PFVAE) and FEP membranes made by the TIPS process are disclosed in U.S. Pat. Nos. 4,902,45; 4,906,377; 4,990,294; and 5,032,274. In the U.S. Pat. Nos. 4,902,456 and 4,906,377 the membranes have a dense surface with either intervals of crack-like openings or pores, either singly, or as a series of several pores. The U.S. Pat. Nos. 4,990,294, and 5,032,274 disclose using a coating of the dissolution solvent on the shaped membrane as it exits the die. In one embodiment, the membrane in a sheet form is stretched in the transverse direction. It was found that the rapid evaporation of the solvent at the high extrusion temperatures gave skinning and poor control of the surface porosity. To overcome the skinning problems, a solvent coating method and post-stretching were employed by previous inventors. In the present invention, the problem of a dense skin and crack-like openings was overcome by the careful control of the air gap at a very short distance, preferably not more than about 0.5 inch, so that a thin skin with uniform surface structure was formed.
It would therefore be desirable to have asymmetric hollow fiber membrane that could operate with very corrosive liquids and gases, and could be used with liquids having surface tensions greater than about 20 mN/m.